after running several hundred cold starts the 100% safe thing to do is on FreqMeas, delay for 40 microsec just after ...begin AND do two dummy cycles before real measurements,
this only costs about 52 microseconds

similarly on FreqCount, do one dummy meas cycle before real measurements (i knew the TLC usually needed this) in my typical application this really has no cost because i just start
my FreqCount loop one time interval early, loop one more time than required, and when pulling results out of the array later just start w index one higher than i would have

in my typical applications these work-arounds are simple and do not stop the show

these cautions are applicable to the TeensyLC and Teensy4, and i have not tested Teensy 3.6, and cannot test any others
i am not sure how to link threads so i will start a new one w this same content titled FreqCount caution.